Field of the Invention
The present invention relates to fiber lasers and amplifiers, and more specifically, it relates to Nd3+ fiber lasers and amplifiers.
Description of Related Art
Fiber lasers and amplifiers are the subject of significant research and have been since University of Southampton demonstrated the potential for low loss rare earth doped optical fibers in 1985 and the subsequently demonstrated gain and lasing in both Neodymium and Erbium doped silica optical fibers. The primary driver of research efforts in optical fiber amplifiers in the late 1980s and early 1990s was the major impact on bandwidth of fiber optic communication systems enabled by wavelength division multiplexing and erbium fiber amplifiers. Optical titer amplifiers enable long haul transmission of many optical channels without the high cost of detecting each individual channel, electronically amplifying and then modulating a laser and recombining the channels every 15-20 km. Instead, a single erbium fiber amplifier restores the optical signal power across all transmission channels in a single compact, efficient and low cost device. C and L band erbium fiber amplifiers provide amplification across 1525 nm to 1620 nm. WDM channel spacing as small as 50 GHz enables a single optical fiber to achieve an information carrying capacity on the order of Tb/s. Early research in erbium fiber amplifiers is well summarized in a number of books specifically on this topic and these amplifiers are now technologically mature.
During the same time period when erbium fiber amplifiers were being developed, significant research and development was also put into developing a rare earth doped fiber amplifier in the 1300-1500 nm telecom window referred to as the O, E and S-bands. However, amplifiers in this wavelength, range have not had the same commercial impact due to efficiency concerns or because they are based upon non-fused silica glasses, which are generally perceived to be more difficult to integrate into the fiber optic network due to differences in the material properties between them and the fused silica material of the rest of the network.
Fiber amplifiers at wavelengths from 1300 nm to 1530 nm fall into several categories. Raman amplifiers are the top contenders and ran attain a wide array of wavelengths as amplification occurs 13.2 THz from the pump wavelength, which can be picked arbitrarily. However, Raman amplifiers require long fiber lengths and high power pump lasers. S-band titer amplifiers based upon erbium and thulium have been studied extensively. In the erbium case, depressed-well fiber geometries are employed to suppress the much higher gain at >1530 nm, but require operating at very high inversions as the emission cross section is significantly less than the absorption cross section at these wavelengths in addition to added losses and fabrication challenges imposed by the depressed-well waveguide design. In the thulium case, research and development efforts have focused on non-fused-silica fibers as the decay from the upper level laser state is faster than the decay from the lower state, making this laser transition self-terminating in fused silica. Recently, bismuth doped fiber amplifiers have emerged as a possible fiber amplifier in the 1320-1360 nm region. However, these amplifiers remain relatively low in optical efficiency and require long fiber lengths. Praseodymium and neodymium were extensively researched for amplification in the E and O band (1300-1450 nm). Praseodymium worked well only in fluoride based fibers.
Neodymium doped fiber lasers and amplifiers in the 1320-1450 nm wavelength range would appear to have some significant attractions as this transition line 4F3/2 to 4I13/2 is a 4-level laser line and thus has no ground state absorption issues. FIG. 1 is a simplified energy level diagram of the relevant Nd3+ transitions. However, this transition also has significant drawbacks that have limited its utility. First, in fused silica as well as other materials, there is a well-known excited state absorption (ESA) from the 4F3/2 state that creates a net optical loss when the laser amplifier is pumped, especially in the region from 1300-1350 nm. The status of the 1350-1390 nm region is less clear as it appears to be convolved with the 1380 nm spectral absorption due to OH. Even with these limitations, net gain (10 dB) and lasing (˜10 mW) has been demonstrated though positive gain occurs well beyond the fluorescence peak where the emission cross-section is highest. The lower cross sections complicate the other key challenge of operation on the 4F3/2 to 4I13/2 transition, which is competition from the preferred transitions 4F3/2 to and 4F3/2 to 4I11/2. It is desirable to provide spectral filtering of the 4F3/2 to 4I11/2 transition to improve performance of a neodymium-doped fiber on the 4F3/2 to 4I13/2 transition.